The invention relates to apparatus for determining the angular displacement of an object by means of a measurement grating which is mechanically connected to the object; and more particularly to such an apparatus which comprises a radiation source for illuminating the measurement grating and a radiation-sensitive detection system comprising a grating-like element.
U.S. Pat. No. 3,973,119 describes such apparatus in which the detection system is a multiple photo cell comprising an array of line-shaped photo-diodes, which are sequentially connected to an electronic circuit by means of an electrical switch. The electronic circuit then processes the signal generated in the photo-diodes. As described in this patent the displacement of an object, for example a part of a tool, can be measured by projecting an image of a first grating which is connected to the object onto a reference grating formed by a multiple photo-cell. The operation of the electrical switch causes an apparent movement of the reference grating over the surface of the multiple photocell. This permits dynamic detection which is substantially independent of environmental conditions, moreover, the direction of the displacement can be determined. The dynamic detection is effected without the use of a separate reference grating and of moving parts for imparting a uniform motion to this grating, so that the displacement transducer is of simple construction and is highly insensitive to vibration. In practice it has been found that the apparatus described in U.S. Pat. No. 3,973,119 is very reliable and accurate.
The line-shaped photo-diodes and the intermediate strips of the multiple photo-cell constitute a grating with parallel grating lines, hereinafter referred to as a "parallel grating". Such a grating is suitable for measuring linear displacements only. For measuring angular displacements of an object, this object would have to be coupled mechanically to a circular disk provided with a grating with radial grating lines, hereinafter referred to as a "radial grating". When a radial grating is imaged onto the multiple photo-cell one line of the radial grating may cover a plurality of photo-diodes, while at the same time parts of the photo-diodes are not illuminated. Therefore, the movement of such a radial grating cannot be measured with suitable accuracy by means of a linear grating.